So many variables, but let's try for a rough guess.
If I take my on photo collection as an example under the untested assumption that it represents an average sample of photos, I can come up with an approximation. I won't pretend to claim it is accurate, but it should give you an idea of how much 35.8 GB really is.
With my iPad plugged into iTunes I see that my photos are taking up .58 Gb of space. Then going into Settings >> About I see that I have 435 photos stored on the iPad. The math tells me this: (approximately, and avoiding the question of binary vs decimal file sizes)
Average photo = 1.3 MB = .0013 GB
1100 photos ~ 1.47 GB
35.8 GB ~ 26,857 photos
The small average size of the photos is probably because they get optimized when importing. If you were to use another app that loaded the full sized images you may get considerably larger files. But they would have to be over 20X larger on average before your 1100 files would fill up your remaining space.
But if you are going to store the original files your best measure it to go to the fold on the computer where they are stored and see how much space they take up.
If you are on OS X, just right click on the folder and choose get info. In Windows 7 do the same, but choose Properties. You are on your own for any other OS.
